Packet structure

Niimbot uses simple TLV packet with preamble, postamble and checksum.

Every field of packet uses big endian.

|5555|c2|01|02|c1|aaaa|
 │    │  │  │  │  └ Postamble (constant)
 │    │  │  │  └─── Checksum (xor sum of type, length and value)
 │    │  │  └────── Value
 │    │  └───────── Length (of data)
 │    └──────────── Type
 └───────────────── Preamble (constant)